The invention relates to an incandescent lamp comprising:
a rotation-symmetrical blown lamp vessel sealed in a vacuum tight manner and provided with: PA1 an axis of symmetry; PA1 a largest diameter transverse to the axis of symmetry; PA1 a neck-shaped first wall portion having a free end; PA1 a second wall portion opposite to the neck-shaped wall portion; PA1 a third mirror-coated inner concave wall portion between the first wall portion and the largest diameter, which wall portion is mainly curved in axial section along the arc of a circle, whose centre of curvature is located on a circle; PA1 a fourth translucent wall portion between the neck-shaped wall portion and the largest diameter; PA1 a filament arranged around the axis of symmetry substantially in a plane transverse to said axis, substantially at the largest diameter; PA1 current supply conductors extending from the filament to contact on a lamp cap connected to the free end of the neck-shaped wall portion.
Such a lamp is known from FR-1 147 918.
Lamps of this kind are designated as bowl mirror lamps. They are intended to be used in an external parabolic reflector, which is in opposition to the mirror-coated wall portion of the lamp. Light, which is generated by the filament and is incident upon the mirror-coated wall portion, is thrown by said wall portion on the parabolic reflector. The reflector concentrates this light and the light which is directly incident from the filament upon the reflector.
In bowl mirror lamps commercially available, the mirror-coated wall portion is spherical. The central part of the mirror-coated wall portion in the immediate proximity of the axis of symmetry may be conical or may not be mirror-coated, however. If the central part is spherical and mirror-coated, the light incident upon it is reflected for the major part to the neck-shaped wall portion and is lost therein. If the central part is not mirror-coated, the incident light is added to the beam formed by the reflector. If the central part is not spherical, but, for example, conical and mirror-coated, incident light is reflected at least in part to the reflector.
In the bowl mirror lamps commercially available, the filament is arranged in a trapezoidal form with open base on one side of a plane passing through the axis of symmetry. This eccentric arrangement of the filament is necessary to prevent that the mirror-coated wall portion produces an image of the filament which falls over the filament. Otherwise, the filament would locally assume a considerably higher temperature, as a result of which the life of the lamp would be shortened.
The eccentric arrangement of the filament has the consequence that all portions (the elements) of the filament are located at a comparatively large distance from the axis of symmetry and hence also at a comparatively large distance from the focus of the reflector, which must be located on the axis of symmetry of the lamp vessel. The light beam formed by the reflector consequently has comparatively low intensity at its centre and is comparatively wide. The beam is also not very homogeneous, which in the case of projection on a screen becomes manifest in a central dark spot.
The object of the lamp according to the aforementioned FR-1 147 918 is to provide a light source, which with a reflector produces a better beam. For this purpose, the third wall portion is curved in axial section along the arc of a circle, whose radius is about 1/4 of the largest diameter of the lamp vessel. The centre of curvature is therefore located together with the associated arc of a circle on the same side of the axis of symmetry. The relevant wall portion can be described as the body of revolution obtained by revolving the arc of a circle about the axis of symmetry. An imaginary circle is then obtained around said axis, on which the centres of curvature are located. The filament is arranged like a crown around said axis and through the centres of curvature. However, it has been found that this lamp does not permit of producing a much better light beam than the lamp commercially available.